The present invention relates to a process of manufacturing a microporous sheet material which may be used as a filter, packaging film, or other uses where a microporous sheet material is desired. The preferred use of the microporous sheet material produced by the process disclosed herein is as a battery separator.
Battery separators are porous diaphragms placed between the positive and negative plates of a battery so that the electrolyte forms the sole internal conducting path between them. Such separators may have smooth surfaces on one or both sides, or have ribs or dimples on one or both sides.
It is considered desirable in the battery separator art to employ the smallest possible pore size since this reduces the danger of active materials being forced through or going through the separator thereby causing shorting between the plates thereof or other detrimental effects. Similarly, small pore sizes are considered to be very desirable for filter materials and some types of packaging materials.
U.S. Pat. No. 3,228,896 discloses a method of making a microporous polyolefin article by molding a plasticized polyolefin into thin sections, and contacting the molded material with a solvent for at least one of the plasticizers employed.
U.S. Pat. No. 3,351,495 discloses a process of producing a battery separator sheet material having a relatively low pore size and satisfactory electrical resistance characteristics. The sheet is made from a high molecular weight polyolefin having an average molecular weight of at least 300,000, a standard load melt index of substantially 0, and a reduced viscosity of not less than 4. The aforementioned sheet is manufactured by extruding the high molecular weight polyolefin in admixture with an inert filler and a plasticizer, and subsequently extracting the plasticizer by the use of a suitable solvent.
U.S. Pat. No. 3,536,796 discloses a process similar to U.S. Pat. No. 3,351,495 but involving a two-stage solvent extraction in which the plasticizer is first extracted with a solvent for the plasticizer and subsequently the filler extracted with a solvent for the filler.
U.S. Pat. No. 4,024,323 discloses a battery separator formed of a blend of polymeric constituents manufactured by extruding the blend together with a plasticizer which is subsequently removed by the use of a suitable solvent.
In the processes described in each of the aforementioned patents, the plasticizer is removed by an extraction process utilizing a solvent for the plasticizer. Such a process requires subsequent separation of the solvent and plasticizer by suitable distillation or other techniques, which necessitates a large capital investment in equipment. In addition, such separation techniques are energy intensive. Furthermore, the commercially attractive processes involve the use of plasticizers soluble in organic solvents, and when organic solvents are employed environmental problems may arise which require expensive techniques to overcome.